Tag: multicellular

Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes. It is meant to complement the usual fare of detailed pieces that are typical for this blog.

Spongebob’s genome reveals the secrets of building an animal

Sponges are animals but, outside of children’s cartoons, they’re about as different from humans as you can imagine. These immobile creatures lie on the very earliest branch on the animal family tree. They have no tissues or organs – their bodies are made of just two layers of cells, twisted and folded into simple shapes. But despite this simplicity, the first complete sponge genome tells us a lot about what it takes to build an animal.

The genome was sequenced from an Australian species called Amphimedon queenslandica by a large team of scientists led by Mansi Strivastava from the University of California, Berkeley. It tells us that sponges share a ‘genetic toolkit’ with humans and all other animals. This includes 4,670 families of genes that are universal to all animals, 1,286 of which separate us from our closest single-celled relatives, the choanoflagellates. Within these families lie the keys to a multicellular existence.

This shared toolkit controls all the fundamental processes that allow individual cells to cooperate as part of a single creature, including how to divide, die, grow together, stick to one another, send signals to one another, take up different functions, and tell the difference between each other and outsiders. They also include many genes that are implicated in cancer, a disease where individual cells go rogue and multiply out of control at the expense of the collective. The presence of cancer-related genes in the sponge genome tells us that as long as cells have been cooperating within a single body, they have needed to guard against the threat of cancer.

Srivastava estimates that the foundations of multicellular life were laid between 600 and 800 million years ago. More than a quarter of the big genetic changes that separate humans from the single-celled choanoflagellates took place during this window, before sponges split off from the ancestors of all other animals. The last common ancestor of all animals emerged during this period and it was a creature of remarkable complexity – a multicellular species that could sense, react to and exploit its environment.

Holy extinction, Batman! One of America’s most common bats could be wiped out in 16 years by new disease

The little brown bat is one of the most common bats in North America but in 16 years, people on the East Coast will be lucky to see any. The bat is being massacred and the culprit is a new disease known as white-nose syndrome caused by the ominously named fungus Geomyces destructans. The fungus grows on the wings, ears and muzzles of hibernating bats, rousing them too early from their deep sleep, sapping their fat reserves and causing strange behaviour.

White-nose syndrome was first identified in a New York cave in February 2006, but it spreads fast. In the last four years, it has covered over 1200 km and contaminated wintering roosts throughout the north-eastern US and its neighbouring Canadian provinces. In infected areas, the fungus is slaughtering bats at a rate of around 45% a year. Cave floors are littered with carcasses.

Five years ago, the little brown bat was thriving, thanks to the installation of bat boxes, conservation efforts and a reduction in pesticide use. The eastern seaboard alone was home to 6.5 million of them. But all of that good is being undone by a single disease. Using a mathematical model, Winifred Frick from Boston University calculated a 99% chance that the species will become locally extinct within 16 years. Even if the current death rate slows to just 5% a year – a highly optimistic target– the population will still collapse to around 65,000 individuals. These last survivors would be just 1% of the previous total, with a 60% chance of dying off by the end of the century. At this stage, the question isn’t if the little brown bat will go locally extinct, but when.

This is just the tip of the iceberg. White-nose syndrome is spreading across North American and at least six other bat species are affected. These animals eat such a large volume of insects that their disappearance would have severe economic and ecological consequences. There’s a desperate need for more research to understand the disease, to keep a track of it, to find ways of fighting it, and to ensure that something like it doesn’t happen again. Frick thinks that white-nose syndrome spread so quickly with such devastating results that it must have been introduced from another part of the world, hitting species whose immune systems were totally unprepared for it. This problem of “pathogen pollution” is a neglected issue in conservation – perhaps the demise of the little brown bat will provide the impetus to take it seriously.

These unassuming fossils may be some of the earliest known examples of complex life on Earth, composed of many cells (like animals and plants) rather than just one (like bacteria). They were uncovered in Gabon by a Abderrazak El Albani from the University of Poitiers, and they’re around 2.1 billion years old. They have been preserved in remarkable detail for their age. They are centimetres in length, and El Albani thinks that they’re probably some of the oldest multi-cellular organisms so far discovered. If he’s right, they’re half a billion years older than the previous record-holders.

Leading a team of 21 scientists, El Albani has painstakingly analysed the fossils. Their three-dimensional structure came with radial slits, scalloped margins and a complicated folded centre. To the team, these complex patterns tell us that the fossils were not simply rock formations. Instead, they were the remnants of once-living things that grew through coordinated signalling between different cells. The fossils are also rich in the mineral pyrite, which is the work of decomposing bacteria; again, this suggests that they were once living.

Back when they were still alive, the Earth was a radically different place. Oxygen made up a small fraction of the atmosphere and a toxic mix of greenhouse gases choked the air instead. Still, things were on the up. The “Great Oxidation Event” was underway, kick-started about 300,000 years previously by tiny bacteria. These microbes pumped oxygen into the air around them as a waste product of photosynthesis, enriching the atmosphere with the gas that would change the planet’s fate. These rising oxygen levels could have been the trigger that allowed multicellular organisms to survive. Without the oxygen, they couldn’t have achieved a large size.

Of course, it’s possible that the fossils could simply be complex colonies of bacteria, rather than true multicellular organisms. El Albani doesn’t rule out that possibility but again, the fossils’ complex three-dimensional shapes don’t quite fit with the idea of a simple bacterial mat. They also contain chemicals called steranes, which often give away the presence of complex eukaryotic cells. But Philip Donoghue from Bristol University, while impressed with the fossils, thinks that we can’t rule out the bacteria idea yet. The steranes, for example, could have moved into the fossils from surrounding rocks. And some scientists aren’t even convinced that the Gabon fossils were once alive.

Bruce Runnegar, who studies the origins of multicellular life, says, “It is difficult to know if this is some unusually complex [non-living] structure, a feature made by a consortium of individual microbes, or evidence for primitive multicellular life.” Some of the fossils’ shapes – such as the wavy surfaces and radial slits – are sometimes seen when different kinds of fluids mix. “The wavy surfaces are unusual, but not unusual enough to convince me to put my money on these structures being “ancient representatives of multicellular life,” he says.

Read more about these fossils, including opinions from several other scientists, at Nature News and an excellent explanation of why we can confidently say that the fossils at 2.1-billion years old at Highly Allochthonous

Sabre-tooth cats wrestled prey with powerful front legs

From looking at the skeleton of a sabre-tooth cat, it would seem obvious what its main weapons are. But impressive though the huge canines are, they’re only part of its arsenal. Its stocky frame and sturdy front legs are equally important. Julie Meachen-Samuels and Blaire van Valkenburgh from the University of California, Los Angeles, have studied the skeleton of Smilodonfatalis, the most famous of the sabre-tooth cats (they’re not tigers). Using a digital X-ray machine at the Smithsonian Institution, the duo showed that its humerus (the bone between shoulder and elbow) was reinforced by extra-thick bone.

Its outer shell, or cortex, was thickened to a greater degree than any other living cat. In terms of sturdiness, it even outclassed the equally extinct but considerably larger American lion. The extra reinforcement made Smilodon’s front legs very difficult to break, bend or compress. These sturdy limbs also had expanded attachment points for the cat’s relatively large muscles. The hind legs, while still thickened, was still within the range of normal variation for other cats.

This new research fits nicely with the modern image of Smilodon as a subtle killer that hunted in a very different way than modern cats. It combined elements of a wrestler and an assassin, grapping large prey to the ground with its powerful front legs, and killing them quickly with a lethal stab of its famous teeth.

Modern cats inflict death more slowly, with a suffocating bite to the throat. But there is no way that Smilodon could have done the same. Its skull and teeth are surprisingly weak, and might have broken during a protracted struggle. Instead, they were probably used to deliver an incisive and fatal bite to the blood vessels of the neck, after the prey had already been pinned. This specialisation allowed it to tackle very large prey, but it might also have been its downfall. As the largest mammals went extinct during the last ice age, Smilodon’s overpowering tactics would have done little good against smaller, more agile targets.

Since the first living things appeared on the planet, the biggest among them have become increasingly bigger. Over 3.6 billion years of evolution, life’s maximum size has shot up by 16 orders of magnitude – about 10 quadrillion times – from single cells to the massive sequoias of today (below right). And no matter what people say, size does matter.

The largest of creatures, from the blue whale to the sauropod dinosaurs, are powerful captors of the imagination, but they are big draws for scientists too. Jonathan Payne from Stamford University is one of them, and together with a large team, he ambitiously set out to understand how the maximum size of living things has evolved throughout the entire history of life on Earth.

Taking each geological era and period in turn, the team scoured the literature for examples of the largest species alive at the time and recorded their size by volume. They also interviewed experts in the field of classification to get their side of the story. Payne’s full database is available online and it showed that the massive increase in life’s maximum size wasn’t a gradual process.

Instead, it happened in two main bursts, which took place in just 20% of the history of life but accounted for 75% of the increase in maximum size. On both occasions, the largest living things became about a million times larger. The first followed the evolution of more complex, compartmentalised cells and the second came after the advent of multi-celled creatures, and both coincided with dramatically rising levels of oxygen in the air. It was a case of environmental changes unlocking pre-existing evolutionary potential.